question on vista and harddrive space

i am sure you guys must have answered this question lots but after i installed vista and now as i am intsalling all my programs and games and what not the space used up does not really match once i was done installing vista all the files it only took up 10g but i looked at spaced used and it was telling me 20gb are being used and so far as i installing before it stayed at 10gbs uncounted for and now its jumped to 20 gbs uncounted for i think i am going crazy and also my sisters laptop has nothing really on it she just uses it for school work and she has a 200 gb HD and now she only has 30gb of free spaces left and she has cleaned out all the files and movies to see if that would free up spaces but nothing seems to be working

any help on this matter would be great thx